1. Field of the Invention
The present invention relates to a method and to a device for determining the state of a vibrating structure of a rotary wing aircraft, particularly a helicopter.
2. Description of the Related Art
More specifically, it is intended to determine whether said vibrating structure, which is subjected to at least one type of vibration, is in a normal state or in an abnormal state (which may be due to a malfunctioning of a component of said structure or of an element of the aircraft that is associated with and/or connected to this structure), while said rotary wing aircraft is operating.
Although not exclusively, the present invention is more particularly applied to determining the state of a system for the suspension of a transmission gearbox of said rotary wing aircraft, such a transmission gearbox being mounted between propulsion means and a lift rotor of said aircraft essentially longitudinally to the axis of said rotor and being suspended with respect to the fuselage of said aircraft. Such a suspension system, which is effective in attenuating vibration in roll, pitch and vertical oscillation, is described in patent FR-2 474 996 and comprises:
struts distributed radially around said transmission gearbox, the upper ends of which are connected to the upper part of the transmission gearbox, and the lower ends of which are articulated to suspension leaves; PA0 a suspension means connecting the lower part of the transmission gearbox to the fuselage of the aircraft and in particular comprising said suspension leaves; and PA0 resonator means associated with said struts respectively. PA0 a) n main vibrational modes of said vibrating structure are defined, n being an integer greater than or equal to 1; PA0 b) p vibrational parameters, for example speeds, displacements, forces, accelerations and/or strains, that can be measured and that will enable the response of said n main vibrational modes to be estimated are defined on said structure, p being an integer greater than or equal to n. To achieve this, and to achieve step c), use is preferably made of a matrix which allows the p vibrational parameters to be estimated from the n fundamental modes and which is determined, either from an identification beforehand, or using known kinematic relationships; PA0 c) estimated values of each of said p vibrational parameters are determined; PA0 d) the values of said p vibrational parameters are measured while said rotary wing aircraft is operating; PA0 e) a correlation coefficient is calculated from said estimated values and said measured values of the p vibrational parameters, preferably complex values; PA0 f) said correlation coefficient is compared with a predefined range of values; and PA0 g) the state of said vibrating structure is deduced from the results of said comparison. PA0 if the correlation (defined by said correlation coefficient) is strong, that is to say if said correlation coefficient lies within said predetermined range of values which is formed accordingly, the only vibrations experienced by said vibrating structure are generated from said main vibrational modes defined in the aforementioned operation a). These vibrations are therefore justified and the vibrating structure is in its normal state; PA0 otherwise, other undefined vibrations are generated in addition, and this reveals an abnormal state of said vibrating structure, these additional vibrations being due in particular to poor operation or a defect of the vibrating structure and/or of an element of the aircraft that is connected thereto. PA0 said correlation coefficient Q is calculated from the expression: ##EQU1## PA0 said predefined range of values is preferably between 0.95 and 1. PA0 sensors arranged on said vibrating structure and capable of measuring the values of said vibrational parameters; and preferably PA0 means which filter the values measured by said sensors, relative to a frequency b.OMEGA., b being the number of blades of the rotary wing aircraft lift rotor and .OMEGA. being its rotational speed; and PA0 a calculation unit receiving the values measured by said sensors and determining, in the aforementioned way, from these and from estimated values of the vibrational parameters, the state of said vibrating structure.